In mobile telephone service of recent years, the demand for data communications has grown in addition to voice call and thus it is important to improve the communication speed. For example, in a GSM (Global System for Mobile communications) system put into widespread use mainly in European and Asian regions, hitherto, voice call has been conducted in GMSK modulation for shifting the phase of a carrier in response to transmission data; EDGE (Enhanced Data rates for GSM Evolution) system for also conducting data communications in 3π/8 rotating 8-PSK modulation (hereinafter, simply 8-PSK modulation) increasing bit information per symbol three times relative to the GMSK modulation by shifting the phase and the amplitude of a carrier in response to transmission data is proposed. A multimode terminal compatible with a plurality of radio systems such as mobile telephone systems of UMTS (Universal Mobile Telecommunications System), GSM, etc., and a wireless LAN (Local Area Network) appears. Further, research on software radio (SDR: Software Defined Radio) technology is also conducted as technology for realizing miniaturization and cost reduction of a radio terminal to cover various radio systems also containing radio systems other than the radio systems mentioned above.
In a linear modulation system involving amplitude fluctuation like the 8-PSK modulation, linearity requirement for a power amplifier of a transmission section of a radio communication device is strict. Generally, the power efficiency in a linear operation region of a power amplifier is lower as compared with that in a saturation operation region. Therefore, if a quadrature modulation system in a related art is applied to the linear modulation system, it is difficult to enhance the power efficiency.
Then, a system of realizing higher efficiency of a power amplifier in linear modulation system, called EER (Envelope Elimination & Restoration), by separating a transmission signal into a constant amplitude phase signal and an amplitude signal, performing phase modulation in a phase modulator based on the constant amplitude phase signal, inputting a constant amplitude phase modulation signal at a level at which the power amplifier performs a saturation operation, and driving a control voltage of the power amplifier at high speed to synthesize amplitude modulation is known (for example, refer to page 427, FIG. 7.1 of non-patent document 1). Particularly, a modulation system of separating a transmission signal in a baseband and performing modulation using a separated constant amplitude phase signal and amplitude signal is called Polar Modulation system (polar modulating system) (for example, refer to page 428, FIG. 7.2 of non-patent document 1). Hereinafter, it will be called polar modulation system to make it clear that a modulation system different from the quadrature modulation system in the related art will be discussed.
In the polar modulation system, in the present technology level, it is difficult to ensure the linearity of output signal amplitude for the input control voltage of a power amplifier for the required dynamic range (hereinafter, simply D-range) to represent an amplitude signal in power amplifier output and therefore it becomes necessary to apply distortion compensation processing technology.
FIG. 29 is a block diagram to show a polar modulation transmitter in a related art incorporating predistortion (hereinafter abbreviated as PD) distortion compensation processing technology described in FIG. 10 of patent document 1.
As shown in FIG. 29, a polar modulation transmitter 20 includes a power amplifier 1, a polar conversion section 2, a distortion compensation processing circuit 3, an amplitude modulation section 10, a phase modulation section 11, and an amplitude phase measurement section 12. The distortion compensation processing circuit 3 includes delay adjustment sections 4 and 5, memory 6, an address generation section 7, an amplitude correction section 8, and a phase correction section 9.
Next, the operation of the polar modulation transmitter 20 in the related art shown in FIG. 29 will be discussed.
To implement a radio communication device transmission section using the polar modulation transmitter 20, the polar conversion section 2 separates a baseband quadrature coordinate signal (IQ signal) input from a signal generation section not shown of a radio communication device into an amplitude signal r(t) and a constant-amplitude phase signal θ(t). Here, r(t) is normalized with a predetermined value.
The distortion compensation processing circuit 3 performs predetermined distortion compensation processing for the amplitude signal r(t) and the phase signal θ(t) and outputs the amplitude signal after subjected to amplitude correction to the amplitude modulation section 10 and also outputs the phase signal after subjected to phase correction to the phase modulation section 11. The configuration and the operation of the distortion compensation processing circuit 3 are described later.
The amplitude modulation section 10 drives the control voltage of the power amplifier 1 based on the amplitude signal output from the distortion compensation processing circuit 3.
The phase modulation section 11 executes phase modulation based on the phase signal output from the distortion compensation processing circuit 3.
The power amplifier 1 combines amplitude modulation with the phase modulation signal output from the phase modulation section 11 based on the output signal from the amplitude modulation section 10 as a control signal.
When a predetermined input signal is given to the polar conversion section 2 and the output signal amplitude of the power amplifier 1 is controlled by decreasing the control voltage at which the amplitude signal becomes a given value at predetermined intervals from the maximum value, the amplitude and phase measurement section 12 measures the output signal amplitude characteristic and the passage phase characteristic of the power amplifier 1 every control voltage value and outputs acquired data to the memory 6.
Next, the configuration and the operation of the distortion compensation processing circuit 3 will be discussed in detail.
To compensate for the delay between the paths of the amplitude modulation signal and the phase modulation signal, the delay adjustment sections 4 and 5 give a predetermined delay to the amplitude signal and the phase signal output from the polar conversion section 2 and output the amplitude signal after subjected to the delay adjustment to the address generation section 7 and the amplitude correction section 8 and also output the phase signal after subjected to the delay adjustment to the phase correction section 9.
The memory 6 stores the inverse characteristics of the output signal amplitude characteristic relative to the input control signal (AM-AM: Amplitude Modulation to Amplitude Modulation conversion, which will be hereinafter called AM-AM characteristic) and the passage phase characteristic (AM-PM: Amplitude Modulation to Phase Modulation conversion, which will be hereinafter called AM-PM characteristic), of the power amplifier 1 in a predetermined input high frequency signal amplitude, output from the amplitude and phase measurement section 12, and outputs an amplitude correction signal and a phase correction signal of the inverse characteristics of the power amplifier 1 in response to an address signal output from the address generation section 7. The above-mentioned characteristics indicate the characteristics when a control voltage in a steady state is supplied.
To distinguish the AM-AM characteristic and the AM-PM characteristic of the power amplifier 1 when a control voltage in a steady state is supplied, each of the inverse characteristics, and the characteristic of the power amplifier at the amplitude modulation operation time from each other, hereinafter the characteristic of the power amplifier 1 acquired using a measurement section of the amplitude and phase measurement section 12, etc., will be called forward characteristic (AM-AM forward characteristic, AM-PM forward characteristic), compensation data stored in distortion compensation processing memory of the memory 6, etc., will be called inverse characteristic (AM-AM inverse characteristic, AM-PM inverse characteristic), and the characteristic of the power amplifier 1 at the amplitude modulation operation time will be called dynamic characteristic (AM-AM dynamic characteristic, AM-PM dynamic characteristic).
The address generation section 7 converts the amplitude signal output from the delay adjustment section 4 into a discrete value having a predetermined range and a predetermined step width, found from the compensation data stored in the memory 6 and compensation accuracy and then generates an address signal to refer to the compensation data stored in the memory 6.
The amplitude correction section 8 makes a correction to the amplitude signal output from the delay adjustment section 4 based on the amplitude correction signal output from the memory 6.
The phase correction section 9 makes a correction to the phase signal output from the delay adjustment section 5 based on the phase correction signal output from the memory 6.
Thus, the amplitude modulation signal and the phase modulation signal previously distorted considering the inverse characteristics of the output characteristics of the power amplifier relative to the input control signal become desired output amplitude and phase by receiving the effect of actual amplitude, phase distortion occurring in the power amplifier, and the linearity relative to the input control voltage can be improved.
The operation description of the polar modulation transmitter in the related art described in patent document 1 is now complete. Hereinafter, the polar modulation transmitter 20 described in patent document 1 will be called the polar modulation transmitter in the related art.
Subsequently, the art required for implementing a multimode radio communication device transmission section using the polar modulation transmitter in the related art will be discussed.
To implement a multimode radio communication device transmission section using the polar modulation transmitter in the related art, distortion compensation processing corresponding to the modulation speed and the required D-range of an amplitude signal varying from one system to another to linearize the operation of a power amplifier.
Next, the art required for implementing a transmission section of an SDR communication device will be discussed.
The SDR communication device can be changed to any desired characteristic and function by rewriting software (reconfiguring) containing the computation processing function of a digital signal processing section, and can be compatible with various communication systems incompatible at the manufacturing time of the device, namely, multimode modulation signal. On the other hand, in the current state of the art, it is difficult to reconfigure an analog circuit of a power amplifier, etc., to change to any desired characteristic and function. Therefore, for example, to use the same power amplifier for multimode modulation signal, a compensation art for a characteristic changing in response to a modulation signal is required.    Patent document 1: International Patent Publication No. 2004-501527    Non-patent document 1: Kenington, Peter B, “High-Linearity RF Amplifier Design”, Artech House Publishers